Oxide coated cathodes for electron tubes



Feb. 23, 1965 REL]; s o ETAL 3,170,772

OXIDE COATED CATHODES FOR ELECTRON TUBES Filed Jan. 5, 1961 UnitedStates Patent 3,170,772 OXIDE COATED CA I'IHBOEDES FOR ELECTRON U SReiji Sato and Taro Ohtsuki, Tokyo, and Akira Suzuki, Kawasaki-shi,Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasaki-ski,Japan, a corporation of Japan Filed Jan. 5, 1961, Ser. No. 80,784 1Claim. (Cl. 29-183.5)

This invention relates to improvements in oxide coated cathodes forelectron tubes.

A conventional oxide coated cathode comprises a core or base of metalcontaining a minor quantity of C, Mg, Al, Si, Cr, W, Ti, U, Th, Zr, Beor other element that can reduce oxides, and an oxide coating. Thereducing el ments as enumerated above serve to reduce the alkaliearthoxides covering the core metal to form an alkaliearth layer having goodthermal emissivity.

It has been found, however, that most of the reducing elements containedin the base metals form boundary layers of insulating compounds betweenthe base metals and the oxide coatings. These boundary layers tend toproduce contact potentials or increase interface resistance, thusimpairing the performance of electron emissive cathodes. It has alsobeen found that the very time when such a boundary layer is formedduring the manufacture of an electron tube is in the stages ofdecomposition of alkaliearth carbonates, the raw materials for formingthe desired oxides, and subsequent aging of the electron tube, when thecathode is heat treated in a gas atmosphere within the tube.

It is, a primary object of the. present invention to provide oxidecoated cathodes for electron tubes having minimum interface resistanceand thus to improve the thermal emissivity of the cathodes.

Another object of the present invention is to provide electron tubes ofimprove performance and extended life.

Other objects and advantages of the present invention, which becomeapparent from the following description with reference to theaccompanying drawings, in which the single figure is an enlargedfragmental sectional view of a cathode embodying the present inventionshowing its laminar construction at a particular stage during itsmanufacture.

Referring to the sole figure, a core metal 1 according to the presentinvention is applied during one step of its manufacture with a very thinlayer of anti-oxidizing metal 2 plated thereon, and an emitter coating 3of emitter materialsof suitable oxides or the raw material ofconventionalcarbonates is formed on the anti-oxidizing layer 2 bypainting or the like. According to the present invention, the core metal1 is formed of nickel containing a minor quantity of a reducing elementor elements selected from the group consisting of C, Mg, Al, Si, Cr, W,Ti, U, Th, Zr, Be and the like that are effective to reduce the oxidesof the coating 3. The preferred metal suitable for forming the thinanti-oxidizing layer 2 is platinum or pure nickel.

When the cathode as above-specified is heated during the course of itsmanufacture, the metal of thin antioxidizing layer 2 is incorporatedunitarily with the core metal 1. In other words, it should be noted thatthe thin anti-oxidizing layer 2 is temporarily present and substantiallydisappears by virtue of the heat treatment during its manufacture toform a substantially uniform structure with the core metal 1.

By appropriately selecting the material of the anti-oxidizing layer 2 aswell as its thickness, the anti-oxidizing 3,170,772 Patented Feb. 23,1965 CCv layer 2 can be so formed that it effectively protects thesurface portion of core metal 1 against oxidization during theexhausting operation of electron tube and its aging. It then disappearswhen a high vacuum has been established in the tube. oxidizing layer 2,it disappears by formation of a solid solution with core metal 1. Whenpure nickel is employed, the core metal 1 and the reducing elementscontained therein diffuse outwardly into the purev nickel antioxidizinglayer 2, thus forming a uniform and indiscriminate core or base on whichusual oxide coating is formed.

Consequently, according to the present invention, the surface portion ofcore metal 1 will not be oxidized during the heat treatment of thecathode, and any substantial boundary layer of insulating or otherundesirable character is not formed between the core metal and oxidecoating. Thus, the interface resistance between the core metal and theoxide coating is extremely low. In addition, after the anti-oxidizinglayer 2 has disappeared as hereinbefore explained, the reducing elementscontained in the base metal can react directly with the oxide coatingwithout obstruction by any intermediary layer. There results aneffective reduction of the oxide layer, so that the lowering of mutualconductance as well as electron emissivity of the vacuum tube is keptsmall throughout the whole life of the tube and eventually extends theuseful life of the tube. In addition, peeling off of the oxide coatingis effectively prevented, and a superior oxide coated cathode withstable performance is obtained.

As practical examples of the present cathodes, core metals 1 of nickelcontaining 0.04% of Mg and 4.0% of W were plated with pure nickel layers2 of l to 5 micron thickness, and painted thereon with conventionalemitter materials such as alkali-earth carbonates forming the emittercoating 3. .Such cathodes were assembled into 616 type vacuum tubes.These tubes were tested in comparison with standard 616 type vacuumtubes of the same ratings underthe same operating conditions for thesame period of working time. The average rate of lowering of mutualconductance of the new tubes was lower than a half of that of standardtubes. When operated under more severe conditions, a more decreased rateof lowering of mutual conductance has been obtained.

What is claimed is:

An oxide coated cathode having a minimized interface resistance for anelectron tube, said cathode comprising a core metal of about 0.04% magnesium, 4% tungsten and the remainder nickel, a layer of pure nickelhaving a thickness between 1 to 5 microns plated on said core metal, anda layer of emitter materials applied over said layer of pure nickelwhereby when said cathode is heated said layer of pure nickel isincorporated with said core metal so that no interface layer existsbetween said emitter material and said core metal. 8

References Cited by the Examiner UNITED STATES PATENTS DAVID L. R ECK,Primary Examiner. WHITMORE A. WILTZ, HYLAND BIZOT, Examiners.

When platinum is used as the anti

